This invention relates to a capacitor bank comprising a plurality of capacitor units for storing energy and for producing a large impulse current, and more particularly to such a capacitor bank provided with a protective device or means for preventing the propagation of the failure of the capacitor units resulting from the destruction of electrical insulation, etc., thereof.
The capacitor banks, which comprise a plurality of capacitor units for storing energy and generate a large current upon closure of a circuit-closure switch, are simple in structure and are capable of producing a large current instantaneously at a predetermined moment of time. Thus, they are widely used, for example, as a power source for supplying pulse or impulse currents for experimental magnetic confinement type plasma devices, etc.
The capacitor banks generally consists of a plurality of capacitor units coupled in parallel to provide for the required capacitance value. If an electrical insulation failure takes place in the capacitor element or the terminals, etc., of one of the capacitor units during the charging operation thereof, for example, the failing capacitor unit is almost short-circuited. As a result, the energy stored in other capacitor units coupled in parallel therewith flows into the failing capacitor unit, which may explode due to the accumulated heat, etc. In the case where the amount of energy stored in the capacitor bank is small, the propagation of the failure may be prevented to some degree by the impedance of the circuit provided by the transmission cables, etc., interposed between the respecctive capacitor units, which impedance suppresses the short-circuit current. In recent years, however, large-sized capacitor banks capable of storing a large amount of energy are becoming increasingly common; when an insulation failure occurs in such large capacitor banks, the impedance of the transmission lines, etc., may prove to be insufficient for suppressing the level of the short-circuit current, and a large amount of energy may flow instantaneouly into the failing capacitor unit, with the result that the failing capacitor unit is eventually exloded or that the transmission lines are destroyed by an overcurrent exceeding the allowable level of the current that can flow therethrough.
Thus, various protective devices are proposed for preventing excess energy from flowing into the capacitor unit under failure. FIG. 1 shows a capacitor bank provided with a conventional protective device disclosed in Japanese Patent No. 738536. The capacitor bank of FIG. 1 comprises a plurality of capacitor units C1 through Cn coupled in parallel for supplying a large pulse current to a load L via a circuit-closure switch SW. The protective short-circuiting devices S1 through Sn, provided for the respective capacitor units C1 through Cn, operate as follows: When one, say Cj, of the capacitor units C1 through Cn undergoes an electrical insulation failure, a large current flows into the failing capacitor Cj, via the associated protective short-circuiting device Sj, from the other capacitor units coupled in parallel with the failing unit Cj. By means of the electromagnetic force generated by this large current, the electrical insulation within the protective short-circuiting device Sj is destroyed so as to short-circuit the two terminals of the failing capacitor unit Cj via a bypass line 9, thereby preventing the current from flowing into the failing capacitor unit Cj. The explosion and fire of the failing capacitor unit Cj is thus avoided.
FIG. 2 shows an example of the structure of the protective short-circuiting devices S1 through Sn, which structure is disclosed, for example, in Japanese Patent Application Laid-Open (Kokai) No. 57-88825. A pair of lead or terminal plates 1 and 3 and a fuse element 2 electrically coupling them are sandwitched between a pair of electrically insulating sheets 4, to be held together between upper and lower holder plates 5 secured together by means of bolts 8. When the associated capacitor unit Cj fails, a large current flows thereinto via the fuse element 2; thus, the fuse element 2 is melted and exploded due to the heat and the electromagnetic force generated by the current, thereby destroying the insulating sheets 4. As a result, the two terminals of the capacitor unit Cj are short-circuited via the lead plate 1, the lower holder plate 6, and the bypassing line 9, and the energy stored in the other capacitor units is prevented from flowing into the failing capacitor unit Cj. Incidentally, the pressure of the explosion is released via the vent hole 7 formed in the upper holder plate 5. The protective short-circuiting device thus provides for the safety. This type of safety system is better than the system which use as a fuse attached to each capacitor. The role of a fuse is to cut the current but a large current is hard to interrupt; on the other hand, the protective short-circuiting device of FIG. 2 is capable of short-circuiting a large current and is easy to produce and low in cost.
However, the above described protective devices for the capacitor bank has the following disadvantage. Namely, since no means is provided for absorbing the energy stored in the capacitor units, the stored energy produces a large short-circuit current, which keeps on flowing until the energy is dissipated by the resistance which the discharging circuit itself includes. Further, due to the short-circuit current, a large transient voltage, which is not generated under the normal operating conditions, may be developed in the circuit-closure switch transmission system or the load, which are coupled across the capacitor bank. This may aggravate the failure. In addition, a large excess current flows through the failing capacitor unit until the protective short-circuiting device functions; thus, deterioration of the unit, such the carbonization of the capacitor element or of the insulation oil thereof, cannot be avoided. Furthermore, a long delay in the operation of the protective short-circuiting unit may result in further aggravation of the failure, such as the explosion of the capacitor bank.